Processes for preparing vinyl ester polymers in the form of aqueous dispersions by free-radically initiated emulsion polymerizations have long been known. Polymer dispersions of this kind find a wide variety of different uses, for example in binders for adhesives, for fiber binding of nonwoven webs, for impregnation or coating of textile fabrics, in aqueous emulsion paints or, after spray drying, as water-redispersible polymer powders, for example as binders in cementitious or non-cementitious applications in the construction industry.
The use properties of aqueous dispersions of vinyl ester polymers are determined and controlled by the polymer composition, by the nature and proportions of auxiliaries, for example protective colloids, emulsifiers and initiators, and to a crucial degree also by the size and size distribution of the vinyl ester polymer particles. The size of the polymer particles influences rheological properties of aqueous dispersions, for example the viscosity thereof, formulability with additives, setting rate, transparency and gloss of a coating film, the water absorption of a binder film, the mechanical strength thereof, for example the wet abrasion resistance thereof.
The size of the polymer particles can be quantified by the weight-average particle diameter Dw or the number-average particle diameter Dn. The polydispersity PD of the size of the polymer particles can be specified via the ratio of weight-average and number-average particle diameter: PD=Dw/Dn.
The prior art discloses various approaches to controlling the size of the polymer particles. For instance, DE 1595402 teaches the use of particular amounts of emulsifiers and/or protective colloids for this purpose. A decrease in the amount of emulsifiers and an increase in the amount of protective colloid lead to polymers having higher mean particle sizes. GB857514 describes processes for preparing polymers in the presence of protective colloids, emulsifiers and acetylenic di-tertiary alcohols, in which “fine particles” are obtained by complete metered addition of the monomers and “coarse particles”, in contrast, by total addition of the monomers at the start of the polymerization. Disadvantageously, emulsifiers are indispensable for this process. DE102009008143 describes the preparation of vinyl acetate-ethylene copolymer dispersions having mean particle diameters Dw of 50 to 500 nm and polydispersities PD between 1.19 and 1.39, by polymerizing in the presence of emulsifiers and optionally protective colloids, using an amount of emulsifiers at least twice as large as that of protective colloids. DE 102006037317 describes coarsely particulate vinyl ester copolymers stabilized by protective colloid and emulsifier. Emulsifier-free dispersions and polymers having particle sizes Dw of less than 500 nm are not obtainable in this way. In addition, DE 102006037317 mentions that commercial, exclusively polyvinyl alcohol-stabilized vinyl acetate-ethylene copolymer dispersions typically have a mean diameter Dw=860 nm and a polydispersity PD=2.26.
DE 1260145 describes processes for emulsion polymerization of olefinically unsaturated compounds in the presence of modified polyvinyl alcohols and optionally emulsifiers, wherein, in the example, the total amount of polyvinyl alcohol is initially charged in the reactor and the total amount of monomer is metered in during the polymerization, which leads to a particle size of about 500 nm. Disadvantageously, the process does not allow controlled preparation of polymers having other mean particle sizes.
DE 1029565 describes processes for preparing aqueous polyvinyl ester dispersions, by which polymers having particle sizes of 500 nm to 15 μm can be prepared in a controlled manner, by using a particular combination of cellulose derivatives and alkali metal salts of oxydiphenyls and salts of inorganic or organic acids. These additional auxiliaries are deleterious to water resistance. Particle sizes less than 500 nm are unobtainable.
DE 10337183 describes the production of polymodal polymer dispersions having a polydispersity PD very much greater than 2, wherein at least two polyvinyl alcohols of different molar mass are initially charged in the reactor and ionic comonomers are polymerized. DE 10035589 describes the preparation of dispersions of high solids content, based on vinyl esters and ethylene without addition of emulsifier in the presence of protective colloid and seed latex. With weight-average particle sizes of greater than 2000 nm, polydispersities PD between 1.44 and 45.0 are obtained.
EP 1215219 describes the use of polyvinyl acetals as protective colloid. The process leads to polymer particles having polydispersities PD very much greater than 1.6. DE 19962566 describes the use of vinyl ester-(meth)acrylic ester copolymers in the presence of polyvinyl alcohol, wherein at least 50% by weight of the vinyl esters is initially charged. EP 1420033 describes the use of ethylene-modified polyvinyl alcohols. The processes of EP 1420033 and of DE 19962566 lead to polymers having mean particle sizes Dw of >700 nm and polydispersities PD of >1.6.
EP 1262465 describes emulsion polymerizations in the presence of protective colloids, wherein vinyl esters are polymerized in a first stage and (meth)acrylic esters in a second stage. In this way, only very coarse polymer dispersions having weight-average particle diameters Dw>2 μm are obtained, but particle sizes smaller than about 700 nm are unobtainable and the polydispersity PD in each case is much greater than 1.6.
EP 1420033 describes processes for preparing protective colloid-stabilized vinylaromatic-1,3-diene copolymers. Disadvantageously, only very coarse dispersions having weight-average diameters Dw>2.5 μm with high polydispersity PD>>1.6 are obtained.
EP 1788001 describes processes for producing polyvinyl alcohol-stabilized polymer dispersions by emulsion polymerization in the absence of emulsifier at temperatures above 100° C. This process too leads to polymer particles having weight-average particle sizes Dw above 800 nm with a broad particle size distribution PD>1.6.
DE 3227090 describes the production of aqueous dispersions by emulsion polymerization of vinyl halides and/or vinyl esters using protective colloids which are not added until after the polymerization has commenced. The particle sizes achieved, which are not specified in more detail in the description, are below about 1 μm with a broad distribution. Reworking of the process described in DE 3227090 in EP 0224169, in experiments in group A, however, gave a weight average of 1.87 μm combined with a polydispersity PD=9.35 in experiment A10 of EP 0224169 and, for experiment A11, a weight average of 1.35 μm combined with a polydispersity PD=7.1. EP 0224169 also showed that the processes described in DE 3227090 cannot be executed reproducibly. EP 0224169 describes finely divided, protective colloid-stabilized vinyl chloride-ethylene copolymer dispersions, the particles of which have not been partly flocculated, and which have weight-average particle sizes—determined in a disk centrifuge—of 150 nm to 500 nm and a polydispersity PD of not more than 1.8, where less than 50% by weight of the polymer particles are larger than 1000 nm. EP 0224169 does not disclose any suitable measures by which the particle sizes of the polymers can be varied in a controlled manner. In addition, polyvinyl halides are viewed in a negative light, and so there is a need to replace polyvinyl halides with other polymer systems, if possible without having to accept a loss of performance properties or other properties.
Overall, a wide variety of different variants of emulsion polymerization are thus known. For the production of protective colloid-stabilized vinyl ester polymers, usually at least a relatively large portion of the monomers and of the protective colloid or even the entire amount of protective colloid are included in the initial charge. The measures known to date, however, generally lead to polymer particles having weight-average particle sizes of greater than 700 nm or 800 nm with polydispersities PD>1.6 or even >2. One reason for this is particle aggregation processes that proceed in the course of the polymerization reaction, well after conclusion of the actual particle formation phase—as discussed, for example, in EP 0224169 under the topic of particle flocculation. The time, extent and result of these particle aggregation processes are dependent on the respective specific conditions of the process: if particle aggregation occurs at relatively low monomer conversion values, the particle aggregates formed have the opportunity to coalesce again during the polymerization to form homogeneous spherical particles; if, however, particle aggregation occurs at a relatively late stage and/or at high monomer conversion values, nonspherical particle aggregates may be present in the end product. These cause, for example, high dispersion viscosities. Overall, the known emulsion polymerization processes for preparation of vinyl ester polymers in the presence of protective colloids offer only limited opportunities for establishment of desired particle sizes, one particular reason being the particle agglomeration processes mentioned. Today, there is also no knowledge as to how finely divided dispersions, especially stabilized by protective colloid alone, having polydispersities PD<1.6 can be produced in a controlled manner.
Moreover, particle aggregates can be deaggregated under mechanical shear stress, which leads to changes in the particle size distribution and the dispersion viscosity. However, changes in such properties of the dispersions are undesirable and should be avoided.
The use of protective colloids, especially polyvinyl alcohols, in polymer dispersions is frequently desirable because of their advantageous performance properties. Unlike emulsifiers, they contribute, for example, to an increase in the mechanical strength of application products, for example in adhesives for paper, packaging or for wood gluing, but also in the case of use as a binder powder, for example in tile adhesives. Emulsifiers, especially emulsifiers of low molecular weight, can migrate within the binder film, which can lead to a change in its properties of the time, for example to a decrease in its strength. Moreover, it is desirable from an environmental point of view to dispense with emulsifiers of low molecular weight, especially with alkylphenol ethoxylates, and to work exclusively with protective colloids. There is consequently still a great need specifically for processes for controlled preparation of protective colloid-stabilized, finely divided vinyl ester polymers having particular settable particle diameters and low polydispersities PD.